Printed circuit board and method for producing a printed circuit board of this type and for producing a laminar composite material for such a printed circuit board

ABSTRACT

A printed circuit board for an electronic circuit, especially for the ultra-high frequencies located in the GHz range that comprises at least one conductor layer, which is arranged on top of an insulating layer and which is flatly joined to said insulating layer. Improved mechanical, thermal and electrical properties are attained by virtue of the fact that the insulating layer is a thin glass layer.

TECHNICAL FIELD

[0001] The present invention concerns the field of electronic circuittechnology. It relates to a printed circuit board in accordance with thepreamble of claim 1 and to a method for producing such a printed circuitboard and a layer composite material for such a printed circuit board.

PRIOR ART

[0002] In present-day electronic circuit technology concerning, inparticular, computer technology and the communication and processing ofdata in communications technology, development is tending toward an evergreater integration and packing density and toward ever higher clockfrequencies, which are normally already in the GHz range. Under thesecircumstances, increasing importance is being attached to the printedcircuit boards or printed circuits in the construction of functionallyreliable and, at the same time, cost-effective circuits. On the onehand, such a printed circuit board, in particular for relatively largeseries, must be simple and cost-effective to produce and process (cut,drill, populate, etc.). On the other hand, the printed circuit boardmust have a sufficient thermal and mechanical stability and the bestpossible thermal conductivity, because a considerable amount of heat isgenerally to be expected owing to the high integration density and highfrequencies during operation. Furthermore, it is desirable for theprinted circuit board to be adapted to the ubiquitously usedsemiconductor components (made of Si or else GaAs) with regard tothermal expansion, because this enables direct mounting of thesemiconductor chips on the printed circuit board (Direct Chip AttachDCA) with all its advantages.

[0003] However, the dielectric properties of the insulation materialused within the printed circuit board are particularly important withregard to the high frequencies. Thus, a printed circuit board used inthe extremely high frequency range should have an insulating layerbetween the conductor layers with the lowest possible (relative)permittivity ε_(r) and a small dielectric loss factor tanδ in order tokeep down the losses that increase with the frequency.

[0004] Finally, at the high clock rates and with very fine conductortracks, it is becoming more and more important for the printed circuitboard material to be distinguished by a high degree of homogeneity inthe dielectric and a high uniformity in the external dimensions (smallthickness fluctuations, etc.) because otherwise, in adjacent regions ofthe printed circuit board, undesirable propagation time differences areproduced during the signal propagation and impair the functionality ofthe circuit constructed therewith.

[0005] All the requirements presented are satisfied only poorly or notat all by conventional epoxide-based substrate materials. Therefore,various proposals have already been made in the past in respect ofusing, for single or multilayer printed circuit boards, insulatingintermediate layers made of a sintered glass ceramic which, at the sametime, have good dielectric properties and are adapted to GaAs circuits,for example, in terms of their thermal expansion coefficient (see e.g.U.S. Pat. No. 6,017,642). However, such sintered glass ceramicsubstrates are complicated to produce and, as ceramic plates, have onlya limited mechanical strength, so that they allow the realization of, inparticular, thin single-layer printed circuit boards only withdifficulty.

[0006] Another proposal relates to the use of a “glass paper”—producedfrom glass fibers—as dielectric for printed circuit boards(JP-A-9208252). Although such a fiber material should be less at risk offracture compared with the glass ceramic, the irregular fibrousstructure of the material results in a local inhomogeneity in thedielectric properties, which can lead to the abovementioned propagationtime problems with high line densities and at high frequencies.

[0007] Finally, a whole class of substrate materials for printed circuitboards is known which comprise fluoropolymers filled with additives(ceramic particles, glass fibers) (see e.g. U.S. Pat. No. 5,149,590).Although such materials, which are commercially available for examplefrom the US company Rogers Corp. under the designations RT/duroid5870-5880 and RO3000, exhibit relatively good and homogeneous dielectricproperties and are therefore well suited to extremely high frequencyapplications, such a material is comparatively expensive on account ofthe complicated production and, moreover, has an unfavorable thermalexpansion coefficient which is significantly higher than that ofsilicon.

SUMMARY OF THE INVENTION

[0008] Therefore, it is an object of the invention to provide a printedcircuit board which avoids the disadvantages of known printed circuitboards and, in conjunction with comparatively simple and cost-effectiveproduction, is distinguished by very good mechanical and thermalproperties, has very good dielectric properties and can thus be used upto extremely high frequencies, and is optimally adapted to thecustomarily used Si semiconductor chips in terms of its thermalexpansion. Furthermore, it is an object of the invention to specify amethod for producing such a printed circuit board.

[0009] The objects are achieved by the totality of the features ofclaims 1, 17 and 23. The heart of the invention consists in providing athin glass layer, as is known principally from the technology of liquidcrystal displays (LCDs), as dielectric in the printed circuit boardbelow the at least one conductor layer comprising conductor tracks. Evenwith a small thickness, such a thin glass layer has a good mechanicalstability, has very advantageous dielectric and thermal properties andis distinguished by a high optical quality, which is manifested inparticular in a high homogeneity of the material and a high degree ofplanarity with small evenness and thickness deviations. The use of athin glass layer as dielectric makes it possible to satisfy all therequirements which are made of printed circuit boards for electroniccircuits with a high integration density and extremely high frequencies.

[0010] A preferred refinement of the invention is characterized in thata respective conductor layer is arranged on both sides of the thin glasslayer and connected to the thin glass layer in planar fashion. In thiscase, one conductor layer or both conductor layers may be structured,i.e. comprise individual conductor tracks. Such a thin glass layerprovided with a conductor layer on both sides has the advantage over theone which is coated on one side that an electronic circuit realizedtherewith is provided with an unambiguously defined volume which ispredominantly filled with the thin glass dielectric and hascorrespondingly favorable properties.

[0011] Preferably, the thin glass layer is pulled from the melt andcomprises a modified borosilicate glass, the thin glass layer having athickness in the range from a few μm to a few mm, preferably in therange between 30 μm and 1.1 mm, and having, at 1 MHz, a relativepermittivity ε_(r) of between 6 and 7, preferably of about 6.2, and adielectric loss factor tanδ of about 9×10⁻⁴ and having, for temperaturesof between 20 and 300° C., a thermal expansion coefficient α₂₀₋₃₀₀ ofbetween 4×10⁻⁶K⁻¹ and 8×10⁻⁶K⁻¹, preferably of about 4.5×10⁻⁶K⁻¹.

[0012] In an advantageous manner, the conductor layers in each casecomprise a metal foil, preferably a Cu metal foil, have a thickness ofbetween 5 and 50 μm, preferably of 18 or 35 μm, and are adhesivelybonded to the thin glass layers in each case by means of a connectinglayer, the connecting layers essentially comprising a resin. Suchresin-coated Cu foils (Resin Coated Foil or RCF) are known from thetechnology of HDI circuits, i.e. printed circuit boards with highintegration densities.

[0013] Preferably, the connecting layers are additionally provided witha silane as adhesion promoter. Furthermore, it is advantageous if thesurfaces of the thin glass layers are pretreated in order to improve theadhesion.

[0014] In the simplest case, the printed circuit board comprises anindividual thin glass layer which is adhesively bonded to metal foils onboth sides. Given a small thickness of the thin glass layer, thisresults in a thin, flexible printed circuit board having very goodelectrical and thermal properties.

[0015] However, it is also conceivable for the printed circuit board tocomprise, in a stack one above the other, a plurality of thin glasslayers adhesively bonded to metal foils and thus to form a multilayerprinted circuit board which may also be provided with plated-throughholes in the customary manner.

[0016] Equally, it is conceivable for the printed circuit board tocomprise, besides a thin glass layer, at least one further insulatingplate made of a different insulating material. The combination ofconductor layer(s) and thin glass layer can thus advantageously beintegrated into a printed circuit board of a conventional type.

[0017] Further embodiments emerge from the dependent claims.

BRIEF EXPLANATION OF THE FIGURES

[0018] The invention will be explained in more detail below usingexemplary embodiments in connection with the drawing, in which

[0019]FIG. 1 shows, in a sectional illustration, the construction of anunstructured printed circuit board in accordance with a first preferredexemplary embodiment of the invention;

[0020]FIG. 2 shows the printed circuit board from FIG. 1 after the(two-sided) structuring of the conductor layers and with a semiconductorchip which, by way of example, is applied directly by means of the DCAmethod;

[0021]FIG. 3 shows, in a sectional illustration, the construction of a(structured) printed circuit board in accordance with a second preferredexemplary embodiment of the invention;

[0022]FIG. 4 shows, in a sectional illustration, the construction of a(structured) multilayer printed circuit board in accordance with a thirdpreferred exemplary embodiment of the invention;

[0023]FIG. 5 shows, in two substeps (a) and (b), a method for producinga layer composite material for a printed circuit board according to FIG.1 in accordance with a preferred exemplary embodiment of the invention;and

[0024]FIG. 6 shows, in six substeps (a) to (f), a method for producing aprinted circuit board according to FIG. 3 in accordance with anotherpreferred exemplary embodiment of the invention.

WAYS OF EMBODYING THE INVENTION

[0025] The invention will be explained below using examples in which athin glass layer is provided with a structured or unstructured conductorlayer in each case on both sides. However, it is also conceivable, inprinciple, within the scope of the invention to provide a conductorlayer only on one side of the thin glass layer. The construction and themethod for producing a printed circuit board or a layer compositematerial with such a thin glass layer “occupied” on one side emergeanalogously from the explanations below.

[0026]FIG. 1 represents, in a sectional illustration, the constructionof an unstructured printed circuit board in accordance with a firstpreferred exemplary embodiment of the invention. In this case, theprinted circuit board 10 comprises an individual thin glass layer 13, aconductor layer 11 and 15 respectively being arranged on the two sidesof said thin glass layer. The conductor layers 11, 15 are formed by Cumetal foils which are adhesively bonded to the thin glass layer 13 inplanar fashion by means of an adhesive connecting layer 12 and 14,respectively.

[0027] Thin glass layers of the kind provided in the context of theinvention and used to achieve the advantages according to the inventionare known from the prior art (see e.g. EP-A1-0 972 632 or DE-A1-198 10325). They have thicknesses of between about 30 μm and 2 mm, are pulledfrom the melt, and are distinguished by a good mechanical and opticalquality and excellent thermal and electrical properties. Such thin glasslayers are produced commercially on a relatively large scale forelectrooptical displays (e.g. LCDs) and their price is thereforecomparatively favorable.

[0028] Suitable thin glass layers or thin glasses are produced andoffered by the company Schott Glas, Mainz(DE) under the typedesignations AF 45 and D 263 T.

[0029] The thin glass AF 45 is a modified borosilicate glass with a highproportion of BaO and Al₂O₃. It is normally in sizes of up to 440 mm×360mm and is distinguished by the following properties: AF 45 Thickness(standard) 50 μm to 1900 μm Transformation point T_(g) 662° C. Thermalexpansion coefficient a₂₀₋₃₀₀ 4.5 × 10⁻⁶K⁻¹ Rel. permittivity ε_(r) (at1 MHz) 6.2 Dielectric loss factor tanδ   9 × 10⁻⁴

[0030] The thin glass D 263 T is a borosilicate glass produced from verypure starting materials. It is likewise available in sizes up to 440mm×360 mm and is distinguished by the following properties: D 263 TThickness (standard) 30 μm to 1100 μm Transformation point T_(g) 557° C.Thermal expansion coefficient a₂₀₋₃₀₀ 7.2 × 10⁻⁶K⁻¹ Rel. permittivityε_(r) (at 1 MHz) 6.7 Dielectric loss factor tanδ  61 × 10⁻⁴

[0031] The two thin glass layers or thin glasses are particularly wellsuited to use in a printed circuit board according to the invention.

[0032] The production of a suitable layer composite material for the(unstructured) printed circuit board 10 in accordance with FIG. 1 ispreferably effected in the manner illustrated in FIG. 5: what is used asa starting point is a thin glass layer 13, whose surfaces are firstlyfreed of adhereing moisture (water) and subsequently pretreated—e.g. bymeans of a corona discharge or the like—in order to obtain a good,permanent adhesion during the subsequent adhesive bonding. In order toproduce the conductor layers 11, 15 on the thin glass layer, use is madeof resin-coated metal foils (Resin Coated Foils RCFs) 28, 29 made of Cu,a connecting layer 12 and 14 respectively being applied to the adhesivebonding side thereof (FIG. 5a). The connecting layers 12, 14 contain aresin which, expediently, is partly cured or prereacted. Suchresin-coated Cu metal foils are known from the technology of large scaleintegrated (HDI) circuits.

[0033] In the context of the invention, particularly suitableresin-coated foils are offered for example by the company Isola AG(DE)under the designation ISOFOIL 160 and RCC. The (ISOFOIL 160) foils havea thickness of the copper foils of. 18 μm or 35 μm, for example, and areprovided with a 75 μm resin layer which is prereacted (so-called B stagecoating).

[0034] The resin-coated foils 28, 29 are packed, with the additional useof a silane layer for adhesion promotion (in this respect, see e.g. U.S.Pat. No. 5,149,590), with the pretreated central thin glass layer 13 toform a stack. The stack is then adhesively bonded under the action ofheat and pressure (FIG. 5b). A layer composite material or an(unstructured) printed circuit board 10 in accordance with FIG. 1 isobtained as a result. The conductor layers 11, 15 can then optionally bestructured using known methods from printed circuit board production, inorder to produce as required specific conductor tracks (or conductorareas) 16, 17 (FIG. 2). By way of example, SMD devices or the like canthen be soldered or conductively bonded onto the structured printedcircuit board 10 in accordance with FIG. 2. A particularly advantageoustype of mounting is made possible by virtue of the thermal expansioncoefficients of the thin glass layer 13 which are adapted to thecustomary semiconductor materials: thus, in accordance with FIG. 2, asemiconductor chip 18 provided with corresponding connection contacts 19can be mounted directly on the printed circuit board 10 (so-calledDirect Chip Attach DCA). As a result of this, even higher packingdensities can be achieved in conjunction with simplified mounting andhigh reliability.

[0035] In addition to the simple printed circuit board 10 from FIGS. 1,2 and 5 which comprises only one thin glass layer 13 and two conductorlayers 11 and 15 and can therefore be made particularly thin andflexible, it is also possible to realize in an analogous mannermultilayer printed circuit boards 30 which comprise, in a stack oneabove the other, a plurality of thin glass layers 13, 13′ and 13″ withconductor layers 34 and 35 lying in between and outer conductor layers31, 38 (FIG. 4). In this case too, each conductor layer is provided witha corresponding connecting layer 32, 33, 36, 37 which adhesively bondsthe conductor layer to the respective thin glass layer. In this case,the inner conductor layers 34, 35 are expediently embedded in anassociated connecting layer 33 and 36, respectively. It goes withoutsaying that such a multilayer printed circuit board 30 may also beprovided with plated-through holes (not shown in FIG. 4) which areproduced in a manner known per se and connect conductor tracks indifferent conductor layers to one another.

[0036] Another possibility within the scope of the invention consists inintegrating a thin glass layer that is provided with a conductor layeron one side or on both sides into a printed circuit board of aconventional type. Thus, the example illustrated in FIG. 3 shows aprinted circuit board 20 in which a central thin glass layer 13 withconductor layers 23, 24 and conductor tracks 27, 27′ bonded on bothsides is arranged between two insulating plates 21, 26 of a conventionaltype (e.g. based on epoxy resin or polytetrafluoroethylene) andadhesively bonded to said insulating plates. Connecting layers 22, 25once again serve here for adhesive bonding, the conductor layers 23, 24being embedded in said connecting layers.

[0037] The production of such a “hybrid” printed circuit board 20 withconventional insulating plates and thin glass layers is illustrated invarious steps in FIGS. 6(a)-(f): the starting point is one of theinsulating plates, namely the insulating plate 21, onto which a firstconductor layer 23 is bonded in a conventional manner by means of afirst connecting layer 39 and is then structured (FIG. 6a). The firstconnecting layer 39 is then filled up whilst covering the firstconductor layer to form the final connecting layer 22 (FIG. 6b). Thestructure 21, 22, 23 thus prepared is thereupon adhesively bonded on oneside to a correspondingly pretreated thin glass layer 13 (FIG. 6c). Afurther structure 24, 25, 26 can then be bonded onto the free side ofthe thin glass layer 13 (FIGS. 6e,f), which structure comprises theother insulating plate 26 provided with a second conductor layer 24. Inthis case, the second conductor layer 24 is bonded onto the insulatingplate 26 by means of a second connecting layer 40, and subsequentlystructured (FIG. 6d), and the second connecting layer 40 is then filledup to form the final connecting layer 25 (FIG. 6e). In this exemplaryembodiment too, it is conceivable to use resin-coated Cu metal foils toproduce the conductor layers 23 and 24.

[0038] Overall, the invention yields a printed circuit board which isdistinguished by the following properties and advantages:

[0039] The printed circuit boards can be made very thin without lossesin mechanical stability

[0040] A direct mounting of semiconductor chips is possible by virtue ofthe adaptation of the thermal expansion coefficients of the centraldielectric (thin glass) to the customary semiconductor materials

[0041] The optical quality, homogeneity and uniformity in the dimensionsof the thin glass material avoid disturbing influences on the signalpropagation, in particular at high frequencies

[0042] The comparatively good thermal conductivity of the thin glasslayer facilitates the dissipation of heat and thus allows higherintegration densities

[0043] The small relative permittivity and the small dielectric lossfactor of the thin glass material enable the circuit arranged on theprinted circuit board to have very high frequencies

[0044] The thin glass layers are commercially available in outstandingquality and at favorable prices from series production and thereforelimit the costs of the printed circuit boards

[0045] The thin glass layers can be worked and processed well in thecontext of printed circuit board production; in particular, knownmethods of printed circuit board production can be used withoutdifficulty

[0046] Multilayer printed circuit boards can be produced withoutdifficulty, which boards can be made even more compact on account of thesmall thickness of the thin glass layers

[0047] Even finer conductor track structures are possible on account ofthe high degree of homogeneity and uniformity of the dielectric.

[0048] The layer composite material produced by the method according tothe invention is particularly well suited as starting material for theprinted circuit board. However, it is also conceivable to use thismaterial in other applications.

LIST OF REFERENCE SYMBOLS

[0049]10, 20, 30 Printed circuit board

[0050]11, 15 Conductor layer (Cu metal foil)

[0051]12, 14 Connecting layer

[0052]13, 13′, 13″ Thin glass layer

[0053]16, 17 Conductor track

[0054]18 Semiconductor chip

[0055]19 Connection contact (semiconductor chip)

[0056]21, 26 Insulating plate (e.g. epoxy)

[0057]22, 25 Connecting layer

[0058]23, 24 Conductor layer (Cu metal foil)

[0059]27, 27′ Conductor track

[0060]28, 29 Resin-coated metal foil (Cu)

[0061]31, 34, 35, 38 Conductor track

[0062]32, 33, 36, 37 Connecting layer

[0063]39, 40 Connecting layer

1. A printed circuit board (10, 20, 30) for an electronic circuit, inparticular for extremely high frequencies in the GHz range, comprisingat least one conductor layer (11, 15; 23, 24; 31, 34, 35, 38), which isarranged on an insulating layer and which is connected to the insulatinglayer in planar fashion, characterized in that the insulating layer is athin glass layer (13, 13′, 13″).
 2. The printed circuit board as claimedin claim 1, characterized in that a respective conductor layer (11, 15;23, 24; 31, 34, 35, 38) is arranged on both sides of the thin glasslayer (13, 13′, 13″) and connected to the thin glass layer (13, 13′,13″) in planar fashion.
 3. The printed circuit board as claimed ineither of claims 1 and 2, characterized in that the thin glass layer(13, 13′, 13″) is pulled from the melt and comprises a modifiedborosilicate glass.
 4. The printed circuit board as claimed in one ofclaims 1 to 3, characterized in that the thin glass layer (13, 13′, 13″)has a thickness in the range from a few μm to a few mm, preferably inthe range between 30 μm and 1.1 mm.
 5. The printed circuit board asclaimed in one of claims 1 to 4, characterized in that the thin glasslayer (13, 13′, 13″) has, at 1 MHz, a relative permittivity ε_(r) ofbetween 6 and 7, preferably of about 6.2, and a dielectric loss factortanδ of about 9×10⁻⁴.
 6. The printed circuit board as claimed in one ofclaims 1 to 5, characterized in that the thin glass layer (13, 13′, 13″)has, for temperatures of between 20 and 300° C., a thermal expansioncoefficient α₂₀₋₃₀₀ of between 4×10⁻⁶K⁻¹ and 8×10⁻⁶K⁻¹, preferably ofabout 4.5×10⁻⁶K⁻¹.
 7. The printed circuit board as claimed in one ofclaims 1 to 6, characterized in that the conductor layers (11, 15; 23,24; 31, 34, 35, 38) in each case comprise a metal foil, preferably a Cumetal foil.
 8. The printed circuit board as claimed in claim 7,characterized in that the metal foils have a thickness of between 5 and50 μm, preferably of 18 or 35 μm.
 9. The printed circuit board asclaimed in either of claims 7 and 8, characterized in that the metalfoils are adhesively bonded to the thin glass layers (13, 13′, 13″) ineach case by means of a connecting layer (12, 14; 22, 25; 32, 33, 36,37; 39, 40).
 10. The printed circuit board as claimed in claim 9,characterized in that the connecting layers (12, 14; 22, 25; 32, 33, 36,37; 39, 40) essentially comprise a resin.
 11. The printed circuit boardas claimed in claim 10, characterized in that the connecting layers (12,14; 22, 25; 32, 33, 36, 37; 39, 40) are additionally provided with asilane as adhesion promoter.
 12. The printed circuit board as claimed inone of claims 9 to 11, characterized in that the connecting layers (12,14; 22, 25; 32, 33, 36, 37; 39, 40) each have a thickness of a few 0.10μm, preferably about 20 μm, between the conductor layers (11, 15; 23,24; 31, 34, 35, 38) and the adjacent thin glass layers (13, 13′, 13″).13. The printed circuit board as claimed in one of claims 9 to 12,characterized in that the surfaces of the thin glass layers (13, 13′,13″) are pretreated in order to improve the adhesion.
 14. The printedcircuit board as claimed in one of claims 1 to 13, characterized in thatthe printed circuit board (10) comprises an individual thin glass layer(13) which is adhesively bonded to metal foils on both sides.
 15. Theprinted circuit board as claimed in one of claims 1 to 13, characterizedin that the printed circuit board (20) comprises, in a stack one abovethe other, a plurality of thin glass layers (13, 13′, 13″) adhesivelybonded to metal foils.
 16. The printed circuit board as claimed in oneof claims 1 to 13, characterized in that the printed circuit board (20)comprises, besides a thin glass layer (13), at least one furtherinsulating plate (21, 26) made of a different insulating material.
 17. Amethod for producing a layer composite material, in particular for aprinted circuit board as claimed in one of claims 1 to 15, characterizedin that a thin glass layer (13) is provided with a conductor layer (11,15; 23, 24; 31, 34, 35, 38) by being adhesively bonded at least on oneside, in planar fashion, to a resin-coated metal foil (28, 29), inparticular a resin-coated Cu metal foil.
 18. The method as claimed inclaim 17, characterized in that the thin glass layer (13) is adhesivelybonded to a resin-coated metal foil (28, 29), in particular aresin-coated Cu metal foil, on both sides.
 19. The method as claimed ineither of claims 17 and 18, characterized in that the layer compositeformed is adhesively bonded to further thin glass layers (13′, 13″) andresin-coated metal foils.
 20. The method as claimed in one of claims 17to 19, characterized in that a thin glass layer (13) is used which (a)is pulled from the melt and comprises a modified borosilicate glass, (b)has a thickness in the range from a few μm to a few mm, preferably inthe range between 30 μm and 1.1 mm, (c) has, at 1 MHz, a relativepermittivity ε_(r) of between 6 and 7, preferably of about 6.2, and adielectric loss factor tanδ of about 9×10 ⁻⁴ and (d) has, fortemperatures of between 20 and 300° C., a thermal expansion coefficientα₂₀₋₃₀₀ of between 4×10⁻⁶K⁻¹ and 8×10⁻⁶K⁻¹, preferably of about4.5×10⁶K⁻¹.
 21. The method as claimed in one of claims 17 to 20,characterized in that the resin-coated metal foils has a metal foilthickness of between 5 and 50 μm, preferably of 18 or 35 μm.
 22. Themethod as claimed in one of claims 17 to 21, characterized in that thesurfaces of the thin glass layer (13) are pretreated in order to improvethe adhesion.
 23. A method for producing a printed circuit board asclaimed in one of claims 1 to 16, characterized in that, in order toform a layer composite integrated into the printed circuit board (20), athin glass layer (13) is adhesively bonded to an insulating plate (21,26) at least on one side, in planar fashion, which insulating plate isprovided with a conductor layer (23, 24) on the adhesive-bonding side.